New Record of Polonium Radiohalos, Stone Mountain Granite, Georgia (USA)

Abstract

The Stone Mountain granite samples in this study yielded no zircon crystals and no diffuse radiation stains. These data are insufficient to determine the timing and mode of formation of these radiohal

Summary

The Stone Mountain granite, Georgia, (USA) is shown to contain
radiohalos of the single-ring type, probably 210Po. Xenoliths collected
from the same site are devoid of radiohalos, but do contain crystalline inclusions
(probably zircons) that are bordered by diffuse radiation stains. The granite
samples in this study yielded no zircon crystals and no diffuse radiation stains.
These data are insufficient to determine the timing and mode of formation of
these radiohalos in the Stone Mountain granitic pluton, so further studies are
warranted.

Figure 1. Location of Stone Mountain, Georgia, USA.

Materials and methods

Palm-sized granite and xenolith samples were collected on the south-west side of Stone Mountain, GA, adjacent to Howell Lake, at the 1,200 foot level above the main wildlife trail, on 20 May 2000 (Figures 1, 2, 3). Biotite schlieren were not collected.1 Samples were gently washed in distilled water to remove extraneous dirt and debris, although microscopic pine and other pollen grains remained (see arrows, Figures 4,5). Water was shaken from the samples and then they were allowed to air dry.

Samples of biotite flakes from the granite (see arrow, Figure
2), and surface biotite flakes on the xenoliths (all sides) were collected under
a dissecting microscope using the ‘sticky-tape’ method.2
In this method, clear sticky tape was pressed firmly into sample areas and rapidly
pulled away, removing thin flakes of biotite with the tapes. Tapes with flakes
of biotite were affixed to glass slides, examined under a compound microscope
for the presence of radiohalos, and were subsequently photographed in brightfield
at various magnifications. About 50 tapes of biotite flakes from the granite
samples and 50 tapes of biotite flakes from the xenolith samples were examined.

Results

Xenolith samples contained no identifiable, circular radiohalos with concentric rings. Radiation stains were observed in xenoliths, but only always associated with microscopic crystal inclusions (probably zircons) (Figure 6). These inclusions were found within the cleavage planes of the crystal structure of the biotites. Radiation stains around these large inclusions were never circular, and never demonstrated concentric rings, but mostly diffusely followed the peripheral contours of the zircons, probably because of the large sizes of the inclusions.

On the other hand, 42% of the biotite flakes from the granite
samples yielded single-ring radiohalos (Figures 4, 5, 7). Radiohalos never
had more than one ring. Ring diameters of 19.2 µm (micrometers) were measured
which correspond to diameters reported for 210Po radiohalos
in biotites.3 Furthermore,
the identity of the radiocenters within these 210Po halos could
not be determined due to them being so tiny as to be virtually impossible
to see.

In some instances very dark and concentrated, essentially
circular in cross-section, radiation stains were observed (Figure 8),
but no zircons or other crystals surrounded by diffuse stains such as
observed in the xenoliths were found.

Discussion

A review of the literature shows that of the 22 localities
containing polonium radiohalos so far reported,4
Stone Mountain granite has, to date, not been subjected to microscopic
evaluation for the presence of polonium radiohalos. Diffuse radiohalos
(radiation stains) have been reported surrounding microscopic zircon crystals
in biotite in Stone Mountain granite,5
but this study is the first record of polonium radiohalos found in this
granite.

The radiation stains surrounding zircon inclusions in xenolith
biotites are certainly the result of alpha-particle emissions from the
zircons (and thus are from alpha-particle– producing daughters in the
238U and/or 232Th decay series), but they are not
well-defined halos with concentric rings. The diffuse, oblong patterns
of the stains are probably because of the large sizes and oblong shapes
of the zircons.

The xenoliths studied represent pieces of country rock which were
incorporated into the Stone Mountain granite during its formation.6
They are composed of biotite-plagioclase gneiss. The metamorphic processes that
produced the gneiss would have been responsible for the formation of the biotites,
but the zircon inclusions within the biotites would have to have been already
present in the protolith (parent sedimentary rock) due to the refractory nature
of zircon. The metamorphic processes were, by definition, hot processes that
would have annealed any radiohalos or radiation stains which may have been present
within the protolith.7 Even after
the metamorphism reached its peak, waning temperatures would have still been
sufficiently high for some time to immediately anneal any damage to the biotites
by alpha-emissions from the included zircons.

Thus, the development of these diffuse radiohalos in the xenoliths
must post-date the metamorphic processes. Hydrothermal fluids containing radioactive
elements could have flowed through the biotite cleavage planes associated with
zircons during the cooling of the granite after these xenoliths had been included
in it, and thus could have supplied the zircons with the U and/or Th responsible
for the diffuse stains. However, this scenario can also be excluded because
radiation damage is not spread along the cleavage planes of the biotite in areas
which contain zircons. Instead, these stains are restricted to areas surrounding
the outer edges of zircons. Thus these diffuse radiation stains around the zircon
crystals in these Stone Mountain xenoliths are probably due to decay, after
granite cooling, of U and/or Th in the zircons that was included in the zircon
crystals when they originally formed.

It is possible that the reason only diffuse radiohalos have been
found in the Stone Mountain xenoliths to date is that only oblong zircons occur
in the xenoliths studied, or perhaps because small zircons or other inclusions,
around which well-defined halos would have formed, have not yet been found. It
is also possible that not enough samples have yet been studied, and that well-defined
halos may yet be found to exist in these xenoliths.

The single-ring radiohalos found in the Stone Mountain granite
appear to be identical to those that form by the alpha-decay of 210Po.
Ion-microprobe mass spectrometry analyses of the radiocenters could potentially
confirm that 210Po is the parent radioactive isotope responsible
for these halos, assuming radiocenters large enough to be analysed can be found
and identified. It is hoped that this technique can be employed in a future
study.

Previous published creationist studies1,6
have concluded that the Stone Mountain granite pluton formed rapidly as
a result of remelting of primordial granite and mixing with melted country
rocks during the late Flood Alleghanian orogenic event. Mineralogical,
geochemical and field data,8,5
such as flow-banding, support a molten origin for the Stone Mountain granite
pluton. Furthermore, the observed 210Po radiohalos in the granite
and the diffuse radiation stains in the xenoliths do not seem to support
a primordial origin for the granite, so further work is required to determine
if the single ring halos found in this granite (probably 210Po)
are parentless or secondary halos. This would include annealing experiments
to help characterize the conditions under which these radiohalos in the
Stone Mountain granite formed.

Acknowledgements

The author thanks Carl Froede for access to Stone Mountain and
for samples. In addition, the author is grateful to the reviewers and to Dr
Andrew Snelling for critical comments.

Newsletter

Thank You!

Thank you for signing up to receive email newsletters from Answers in Genesis.

Whoops!

Your newsletter signup did not work out. Please refresh the page and try again.

Answers in Genesis is an apologetics ministry, dedicated to helping Christians defend their faith and proclaim the gospel of Jesus Christ effectively. We focus on providing answers to questions about the Bible—particularly the book of Genesis—regarding key issues such as creation, evolution, science, and the age of the earth.